can: cc770: store echo skb before starting the transfer

On slow systems and high CAN bitrates, the error message
"can_put_echo_skb: BUG! echo_skb is occupied!" did show up because
can_put_echo_skb() was called after starting the transfer.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>